Chlorine atoms are important oxidants at dawn in the marine boundary layer where a variety of organics are also present, including alkenes. Using the relative rate technique, the kinetics of the gas phase reactions of atomic chlorine with a series of alkenes, relative to n-heptane as a reference, have been investigated at (298 AE 3) K and 1 atmosphere in either synthetic air or nitrogen. The rate constant for n-heptane, relative to n-butane whose rate constant was taken to be 2.18 Â 10 À10 cm 3 molecule À1 s À1, was also measured and found to be (3.97 AE 0.27) Â 10 À10 cm 3 molecule À1 s À1 (2s). Based on this value for the n-heptane reaction, the following absolute values for the rate constants, k (in units of 10 À10 cm 3 molecule À1 s À1) for the chlorine atom reactions were determined: propene, 2.64 AE 0.21; isobutene, 3.40 AE 0.28; 1-butene, 3.38 AE 0.48; cis-2-butene, 3.76 AE 0.84; trans-2-butene, 3.31 AE 0.47; 2-methyl-1-butene, 3.58 AE 0.40; 2-methyl-2-butene, 3.95 AE 0.32; 3-methyl-1-butene, 3.29 AE 0.36; 2-ethyl-1-butene, 3.89 AE 0.41; 1-pentene, 3.97 AE 0.36; 3-methyl-1-pentene, 3.85 AE 0.35; and cis-4-methyl-2-pentene, 4.11 AE 0.55 (AE2s). The errors reflect those in our relative rate measurements but do not include the 10% error in the absolute value of the n-butane rate constant upon which these rate constants are ultimately based. A structure-reactivity scheme is presented that assumes that rate constants for addition of chorine atoms to the double bond, as well as that for abstraction of an allylic hydrogen atom, depend upon the degree of alkyl substitution at the double bond and allylic carbons. The surprising result is that the allylic hydrogen atoms react less rapidly with chlorine atoms than the analogous alkyl hydrogens in alkanes. The atmospheric implications for loss of alkenes in the marine boundary layer are discussed.

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The reaction kinetics of atomic chlorine with a series of biogenic hydrocarbons, including the two enantiomers of ␣-pinene, were studied at 298 K and 1 atm pressure using a relative rate technique. The simultaneous losses of the biogenic of interest and a reference compound, either n-nonane or n-butane, were followed using gas chromatography with flame ionization detection as a function of the extent of photolysis of a chlorine atom precursor. Thionyl chloride, trichloroacetyl chloride or in a few trials, acetyl chloride, were photolyzed at 254 nm to generate chlorine atoms, since molecular chlorine reacted in the dark with these organics. The relative rate constants for ethane and isoprene determined relative to n-butane using SOCl 2 and CCl 3 COCl were compared to those determined using Cl 2 to check for possible artifacts. The average relative rate constants for ethane and isoprene (both relative to nbutane) using these new sources are (0.281 Ϯ 0.021) and (2.49 Ϯ 0.39) (Ϯ2 ) respectively, within experimental error of those measured using Cl 2 as the chlorine atom source. The relative rate constants averaged over all sources including Cl 2 are (0.277 Ϯ 0.025) for ethane and (2.42 Ϯ 0.45) for isoprene. The ratios of rate constants for the chlorine atom reactions with the biogenics with formula C 10 H 16 relative to n-nonane were as follows: (

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